We examined the gastrointestinal parasite fauna of adult double-crested cormorants from breeding colonies on 4 very different lakes spanning a major ecotone from prairie to boreal forest in Saskatchewan, Canada. Our objectives were to document regional parasite fauna and identify potential differences based on breeding colony location. Eight species of helminths were identified from the stomach and small intestinal contents of 92 cormorants collected during the summers of 2006 and 2007: 2 nematodes—Syncuaria squamata and Contracaecum cf. rudolphii; 2 trematodes—Ribeiroia ondatrae and Drepanocephalus spathans; and 4 cestodes—Ligula colymbi, Schistocephalus solidus, Paradilepis caballeroi, and Paradilepis sp. Our results include an apparently undescribed species of Paradilepis, new host records for L. colymbi, S. solidus, and R. ondatrae, and new geographic records for S. squamata, D. spathans, and P. caballeroi in cormorants of western Canada. Anisakids, C. cf. rudolphii, were present in total stomach contents of all but 1 cormorant and were variably associated with mucosal ulceration and hemorrhage. The occurrence of an acanthocephalan, Pomphorhynchus bulbocolli, and plerocercoids of the cestode Diphyllobothrium are considered artifacts of a piscivorous diet. Overall, parasite fauna was similar at 3 of 4 lakes; however, birds at a breeding colony that consumed a high proportion of amphibians (instead of fish) had a low prevalence and intensity of D. spathans. Parasite abundance and diversity may be a useful indicator of differences in feeding ecology among wildlife populations.